Kinetics of Mn₂O₃–Mn₃O₄ and Mn₃O₄–MnO Redox Reactions Performed under Concentrated Thermal Radiative Flux

Manganese oxide based on the thermochemical cycle for splitting water is considered using concentrated solar energy. The high-temperature endothermic step is the thermal reduction of Mn2O3, which proceeds at above 1835 K via two sequential chemical reactions from Mn2O3 to Mn3O4 and then to MnO. The kinetic mechanisms of both reactions are investigated by a solar-driven thermogravimeter, with reactants directly exposed to high-flux thermal irradiation. With this arrangement, the overall kinetic rate laws are derived under similar heat- and mass-transfer characteristics existing in highly concentrating solar systems, such as solar towers or parabolic dishes. The experimental results suggest a nth-order rate for the conversion of Mn2O3 to Mn3O4 and a diffusion-controlled regime for the conversion of Mn3O4 to MnO. Activation energies for both reduction steps are determined and compared to previous reported data.